1. Field of the Invention
The present invention relates to a method of reducing nitrogen oxides efficiently from the exhaust gases of an internal combustion engine in an automobile, etc., a plant for producing nitric acid, or the like, and a catalyst used for carrying it out.
2. Description of the Prior Art
The exhaust gases of an internal combustion engine in an automobile, etc., a plant for producing nitric acid, or the like contains harmful components of nitrogen oxides (NO.sub.X) which give rise to environmental pollution. Attempts have, therefore, been made in various fields of industry to reduce nitrogen oxides from such exhaust gases.
There is known a method which employs a catalyst for reducing nitrogen oxides form exhaust gases. According to this method, exhaust gases are brought into contact with a catalyst, whereby the nitrogen oxides contained therein are adsorbed on the surface of the catalyst and are decomposed to nitrogen and oxygen. The oxygen is then reacted with a reducing substance, such as carbon monoxide or hydrogen.
The catalyst which has hitherto been employed for reducing nitrogen oxides comprises a metal, such as copper, palladium, platinum or rhodium, loaded on a support formed from a porous material, such as alumina, zirconia or zeolite, as disclosed in e.g. Japanese Laid-Open Patent Specifications Nos. 11063/1976, 23474/1976 and 86693/1978.
The known method has, however, not been able to remove nitrogen oxides effectively in an oxidizing atmosphere which contains a greater amount of oxygen than is required for oxidizing a reducing substance (such as ammonia, carbon monoxide or hydrogen) completely to water (H.sub.2 O) or carbon dioxide (CO.sub.2). This is because the oxygen which the atmosphere contains tends to react with the reducing substance more quickly than the oxygen which has been separated from the nitrogen oxides and prevents the latter from reacting with the reducing substance effectively. Such an oxidizing atmosphere is, for example, produced by an automobile engine if it is supplied with a fuel-air mixture having a higher ratio of air to the fuel and containing a greater amount of oxygen than is required for the complete combustion of any unburned fuel.